Polyploid cells contain more than two complete sets of homologous chromosomes. Polyploid cells occur both naturally and pathologically. Several vital organs, including the heart, liver and placenta contain polyploid cells. Further, many cancers exist in a polyploid or near polyploid state. Despite the prevalence of polyploidy, its biological implications remain unclear. One model is that extra sets of chromosomes in polyploid cells buffer the effects of genome damaging agents, thus making polyploid cells more tolerant of DNA damage. When subjected to DNA damage, diploid cells activate a canonical response to prevent cells with DNA damage from entering the cell cycle. In the absence of this response, cells enter mitosis with damaged DNA and frequently die via a poorly characterized cell death process known as mitotic catastrophe (MC). In contrast to diploid cells, little is known about how a polyploid cell responds to DNA damage. My objective is to examine how polyploid cells respond to DNA damage. Previously, our lab found that unlike many polyploid cells, Drosophila rectal papillar (hereafter: papillar cells) cells undergo mitotic proliferation. Thus, Drosophila papillar cells allow me to investigate the effects of DNA damage on naturally occurring mitotic polyploid cells. Compared to diploid cells, our lab has found that papillar cells show elevated rates of chromosomal instability, suggesting that they lack a canonical DNA damage response. I have found that distinct types of DNA damage elicit distinct, non-canonical responses in papillar cells. Specifically, papillar cells are highly tolerant of X-irradiation induced DNA damage but die via non-canonical (caspase-independent) MC following aberrant DNA replication. Therefore, my central hypothesis is that polyploid mitotic cells employ non-canonical mechanisms following DNA damage. In Aim 1 I will determine the non-canonical mechanism by which polyploid papillar cells survive irradiation induced DNA damage. I expect to uncover the basic mechanism by which cells stripped of canonical DNA damage responses can survive high levels of irradiation induced DNA damage. In Aim 2 I will determine the non-canonical mechanism by which polyploid papillar cells undergo cell death in response to re-replication induced DNA damage. This research should shed light on the poorly understood mechanisms of MC, a cell death mechanism that is crucial in cells lacking canonical DNA damage responses. Taken together, my proposed research will identify specific mechanisms by which polyploid papillar cells respond to DNA damage. The biological implications of polyploidy remain largely unstudied. I have found that one main difference between polyploid cells and diploid cells is their response to DNA damage. Since both lack of canonical DNA damage responses and polyploidy are recurring features in numerous cancers, understanding distinct ways by which polyploid cells respond to DNA damage is crucial in treatment of polyploid cancers.